1. Field of the Invention
The invention generally relates to co-axial transmission systems for transmitting both telephony and video signals to subscriber locations and in particular to pair gain test units for use therein.
2. Description of the Related Art
Co-axial cables have been widely deployed for providing video signals, such as cable television (CATV) signals, to subscriber locations, i.e. homes or offices. Currently, systems are being developed for also providing telephony signals over the co-axial cables, such as signals carrying telephone calls, facsimile transmissions, Internet data communications and the like. One such system, Mediaspan.TM., is currently being developed by DSC Communications, the assignee of rights to the present invention.
With systems for providing both video and telephony signals over a single co-axial cable, the single cable thereby carries both downstream signals (i.e. signals sent to the subscriber location) and upstream signals (i.e. signals sent from the subscriber location). The downstream signals include both video and telephony signals. The upstream signals typically include only telephony. In some systems, though, upstream signals additionally include upstream video signals such as may be required with interactive television systems.
Separate transmission frequencies are used to distinguish the downstream signals from the upstream signals. The frequency range of 450 to 750 MHz is employed for downstream signals and the frequency range of 5 to 50 MHz is employed for upstream signals. Discrete multiwave tones (DMT) may be employed to transmit the upstream signals, particularly so as to overcome noise problems inherent in upstream co-axial cable transmissions. Such noise problems occur, for example, as the result of the presence of noise sources within the 5 to 50 MHz band (such as motors, washers, compressors and the like) operating near the downstream end of the co-axial cable.
Separate transmission frequencies are also used to distinguish downstream telephony signals from downstream video signals and to distinguish upstream telephony from upstream video signals, if any. Moreover, as to the telephony signals, otherwise conventional pair gain techniques may be employed to permit simultaneous transmission of two or more telephony channels both upstream and downstream to thereby permit, for example, two separate telephone conversations to proceed simultaneously using two separate telephones at the subscriber location. The signals carried on the telephony channels are typically encoded digitally for transmission using, for example, T1 framing.
When implementing such a combined video/telephony co-axial cable transmission system, a co-axial termination unit (CTU) may be provided at each individual subscriber location, with hundreds or perhaps thousands of CTU's connected to a single combined video/telephony co-axial cable. Each CTU is connected to the combined video/telephony co-axial cable via a tap. The CTU is also connected to both the upstream end of a video-only co-axial cable connected into the subscriber location and the upstream ends of any telephone lines that are also connected into the subscriber location. The video-only co-axial cable is typically connected to a television set or video cassette recorder (VCR) at the subscriber location. Tip and ring lines of the telephone lines are typically connected to a telephone, facsimile machine or modem at the subscriber location.
Thus CTU provides an interface between the combined video/telephony co-axial cable and the video-only co-axial cable and separate telephone lines connected into a single subscriber location. To this end, the CTU includes components for converting radio frequency (RF) digital telephony signals received on the telephony channels of the combined video/telephony co-axial cable to analog telephone signals for coupling to the tip and ring lines of the subscriber telephone lines. Likewise, the CTU includes components for converting analog signals received from the tip and ring lines to digital RF signals for transmitting over the combined video/telephony co-axial cable. A modem and a coder-decoder (CODEC) may be employed to handle the conversions. Also the CTU includes circuitry for routing the video signals received from the combined video/telephony co-axial cable to the video-only co-axial cable routed into the subscriber location. The video-only co-axial cable is referred to herein as a "video-only" cable only because, in use, it carries only video signals. The video-only co-axial cable is, however, an otherwise standard co-axial cable which could carry other signals as well.
The components of the CTU are powered by a power signal, typically provided at 90 Volts, carried along the combined video/telephony co-axial cable, i.e. the combined video/telephony co-axial cable is a "wet" cable. Components of the CTU ensure that power is not forwarded onto the video-only co-axial cable connected into the subscriber location and that only standard POTS (i.e. plain old telephone system) power voltages are coupled to the telephone lines. In this manner the presence of the CTU is entirely transparent to the operation of the televisions, VCR's, telephones and the like at the subscriber location which can thereby operate as if receiving signals from standard CATV cables or POTS telephone lines.
An upstream end of the combined video/telephony co-axial cable is connected via an appropriate interface system into a telephone company central office (CO) provided with switching equipment for routing telephone signals to and from the public switched telephone network (PSTN). The interface system receives telephone signals from the PSTN via the CO and also receives video signals from a suitable video source, such as a CATV service provider or a satellite dish, and combines those signals onto the combined video/telephony co-axial cable for transmission to the CTU.
The CO also includes pair gain test equipment for testing the telephony channels carried over the combined video/telephony co-axial cable. Pair gain tests are typically performed periodically to verify that all telephony channels are operational. Also, pair gain tests are performed whenever a problem report is received, such as if a subscriber complains that telephone service has been disrupted. To permit the CO to perform the pair gain test, the CTU must have appropriate equipment for receiving pair gain test signals from the CO over the telephony channels of the combined video/telephony co-axial cable and for responding with appropriate reply signals. Problems arise, however, when implementing pair gain test equipment within CTU's that provide two or more output telephone lines to the corresponding subscriber location. Because a separate CTU is provided at each separate subscriber location, it is important that the costs of each CTU be minimized and that the reliability of the components of each CTU be maximized. However, the need to provide separate pair gain test equipment within each CTU for each separate output telephone line provided by the CTU increases costs and also increases the chances that the pair gain test equipment of any particular CTU may become defective requiring on-site maintenance. Accordingly it would be desirable to provide an implementation of pair gain test equipment within a CTU that reduces costs and increases reliability and it is to that end that aspects of the present invention are primarily directed. It should be noted that the systems described in this Background section are not necessarily prior art to the present invention.